Originally Posted by
subgrade
There are different kinds of thieves: the opportunists who see an unlocked bike and ride off with it; the pros who look for expensive bikes and are equipped for the job and plan ahead; and the regulars, who have the intention of stealing bikes, but aren't going into great risk for that. The latter ones will have one or several tools to cut through weak locks in seconds, but won't bother with U-locks or other serious locks, and although I have no way of knowing, I'd guess that these make up the majority of bike thefts.
I'd guess that the opportunists are the majority. But having said that, knowing your location and crowd makes a big difference. "Regulars" are very unlikely to be patrolling around a small town, or even low traffic places in the suburbs, looking for bikes to steal, so a low security lock would be fine there. Similarly, if you're leaving your bike locked next to a bunch of unlocked bikes, just about any thief is going to go for the low hanging fruit.